For those unfamiliar with Houdini, he had the utmost respect for magicians/illusionists and folks who did tricks to entertain and mystify crowds. But he despised those same practitioners who would take advantage of people in any respect and in his later life, he set about exposing a number of charlatans and giving away tricks to shield people from being duped. He was famous for de-bunking seances and mediums who could conjure ectoplasm -- not because he wanted to expose the trick (he respected how they did it) but because these people were taking advantage of grieving spouses and children and charging top dollar to contact their dead loved ones.
This book focuses a great deal on fire tricks (fire eating, heat resistance, etc.) which date back to pre-historic times and were used as proof that someone was superhuman or godlike because mortal are not immune to fire. He mostly takes umbrage with the fire illusionists of his day because they were quacks -- not only could they pull off impressive feats but they also were selling their elixirs for burns and healing products that were, in actuality, junk science but the performances that they gave lended undue credibility.
I would agree with other reviewers that this book is VERBOSE and very long-winded, although excellently researched. It's a bit of a slow read, but full of interesting tips on how anyone could be "fire resistant" if they had the same arsenal of trickery.